The present invention relates to a polyurethane foam for vehicles that is used, for example, for vehicle parts of cars such as a hood silencer or a dash silencer, is lightweight, exhibits low combustibility, and shows suppressed discoloration, and to a method for manufacturing the same.
A polyurethane foam with which a flame retardant is blended to provide low combustibility has been used for vehicle parts for cars such as interior materials. It has been desired that this polyurethane foam have an apparent density of 20 kg/m3 or less in terms of weight saving. Glass fiber is used for a vehicle part such as a hood silencer formed in the inside of a hood and a dash silencer used as a partition between an engine compartment and a passenger compartment. However, although glass fiber has excellent flame retardancy, it has a high specific gravity (density) and is heavy. Therefore, a hood silencer and a dash silencer using a polyurethane foam which is lighter than glass fiber are called for. Japanese Laid-Open Patent Publication No. 2004-43747 discloses a urethane foam (polyurethane foam) containing expanded graphite and a phosphorus-based flame retardant as a material for a flame-retardant heat-insulating acoustic material.
In order to improve the fuel consumption or the like, the weight of a car is desired to be reduced. Polyurethane foam used for vehicle parts such as a hood silencer is also expected to have a lower density while maintaining the physical properties. Conventionally, it has been difficult to reduce the apparent density of polyurethane foam to 20 kg/ma or less. Specifically, in order to manufacture polyurethane foam having an open cell structure, a predetermined hardness, and a low density, it is necessary to increase the content of water as a blowing agent in a raw material of the polyurethane foam. In this case, the increase in the content of water causes an increase in the exothermic temperature of the raw material to 170° C. or higher. As a result, there is a possibility of self-ignition based on oxidation deterioration (scorching) of polyurethane, and the resulting polyurethane foam is discolored according to the scorching. In order to avoid such a situation, it is known that a blowing auxiliary is added to the raw material with the conventional content of water. As a blowing auxiliary, methylene chloride and liquefied carbon dioxide may be used.
However, methylene chloride is one of the substances that have an adverse influence on environment, and its use is regulated. On the other hand, foaming with liquefied carbon dioxide requires equipment exclusive for supplying liquefied carbon dioxide at a high pressure, and in order to perform foaming smoothly, not only manufacturing conditions are limited but also manufacturing costs are increased. Japanese Laid-Open Patent Publication No. 2006-63296 discloses a technique in which an inorganic compound hydrate is added to a polyurethane-foam raw material for the purpose of endotherm. As the inorganic compound hydrate, for example, iron sulfate hydrate may be used.
However, in order to reduce the density of the polyurethane foam disclosed in Japanese Laid-Open Patent Publication No. 2004-43747, it is necessary to increase the content of water as a blowing agent. As mentioned above, this promoted foam-forming reaction of a raw material increases the exothermic temperature of the raw material, and, as a result, discoloration occurs in the foam due to scorching. The polyurethane foam disclosed in Japanese Laid-Open Patent Publication No. 2006-63296 has an effect in the reduction of the exothermic temperature at the time of reaction and foaming of a raw material. However, since a flame retardant is not blended with the foam raw material, there is a problem that the resulting polyurethane foam does not suppress the combustibility. Therefore, a polyurethane foam for vehicles that exhibits low combustibility and suppression of discoloration with sufficient balance has been desired.